The invention relates to electronic surveillance systems for loss-prevention and security, and in particular, to a detection method and apparatus used in a retail store for automatically determining if a shopping cart, passing through a check-out aisle, contains items on its bottom or lower tray. The invention automates a task that is typically required of a human check-out attendant. The invention may be used in a variety of ways, for example, to direct a visible or audible message to the cashier and/or customer, or to trigger a second system to physically prevent further movement of the cart through the check-out lane.
Shopping carts, as used in supermarkets, for example, often include a bottom tray below the main storage basket for carrying additional items. The bottom tray is a convenience for the customer, and increases the cawing capacity of the carts. However, due to the obscuring presence of the check-out desk, goods carried on the bottom tray tend not to enter the check-out attendant""s lines of sight for long, if at all, and consequently may leave the store unnoticed and unpaid for.
A variety of devices exist to assist check-out attendants in noticing the presence of goods on the bottom tray of the shopping cart. The most common are mirrors positioned so as to afford the attendant an improved view of the lower cart region for at least a short period of time. However, this approach requires the attendant to look at the proper moment, which is not always practical.
A number of devices that automatically detect a shopping cart and indicate the presence of objects on the bottom tray when it passes a fixed location are described in the prior art.
U.S. Pat. Nos. 4,327,819 and 4,723,118 describe detection means responsive to the weight of objects placed on the bottom tray of the cart.
U.S. Pat. Nos. 3,457,423, 3,725,894, 4,338,594, 5,485,0,06, 5,495,102, 5,610,584, 5,500,526, and 5,910,769 describe detection approaches based on various arrangements of discrete phototransmitters, photoreceivers, retroreflective markers, and processing logic for signal sequence recognition and time-delay gating. The basis of all of these approaches is to discriminate the presence of a shopping cart (as opposed to any other passing object) and then to check for the presence of an item on the bottom tray. The cart-detection function in some of these approaches involves the detection of a special tag affixed to the cart, or in others the statically coded combination of a plurality of discrete sensor outputs. The item detection in each of these approaches is either inferred from the blocking of an optical signal, or the backscattered reflection of an optical signal.
U.S. Pat. Nos. 4,237,483 and 5,883,968 describe devices that employ imaging and automated image analysis to detect the presence and type of goods on the bottom tray. In particular, U.S. Pat. No. 5,883,968 describes the use of a digital image analysis technique whereby a reference image of an empty cart is compared to acquired images. It also describes the use of color-discriminating and Identification Code discriminating techniques.
The performance of any detection system may be quantified statistically in terms of its False Detection Rate (FDR), which is the percentage of false detection instances recorded in a statistically significant population of trials. False detection rates can be sub-divided into the xe2x80x9cfalse-positivexe2x80x9d and xe2x80x9cfalse-negativexe2x80x9d type. None of the systems described in the prior art are likely to exhibit zero FDR, because of assumptions and approximations they each make relating to such factors as cart geometry and motion, optical and geometric properties of the items to be detected.
For example, all of the cited prior art that employ discrete, narrow-field photo-detectors depend on uninterrupted cart motion past the sensor array to provide sufficient continuity (coverage) in the scanning phase of the item-detection. However, in practice, shopping carts can and do pause for varying periods of time, and sometimes even reverse direction temporarily before proceeding through the checkout.
The apparatus described in U.S. Pat. No. 5,883,968 employs a two-dimensional imaging sensor (digital camera), which affords an instantaneous view of the whole under cart area thereby providing an advantage over narrow-field photo-detectors. However, the accuracy of the item-detection is adversely impacted by a number of factors including variability of lighting conditions and variability of the proximity of the cart to the camera lens. This latter issue is particularly acute due to the practical necessity of wide-angle lens optics, which suffer from exaggerated perspective distortion. Moreover, automated image analysis algorithms typically require considerably more processing power than the simple logic processing used with discrete photo-detector solutions, thereby increasing the cost of a product.
A means of improving the ability of two-dimensional imaging sensors to discriminate the shape and position of objects in three-dimensional space through the use of structured illumination is taught in U.S. Pat. No. 4,979,815.
The present invention improves on the prior art, by improving the performance of both the cart-detection function and the item-detection function. The former is accomplished without the use of special cart-affixed tags or error-prone static logic, and the latter is achieved through the use of structured illumination imaging and pattern analysis.
The disclosures of all patents/applications referenced in this specification are hereby incorporated herein by reference in their entirety.
In a preferred embodiment of the present invention there is provided an apparatus for detecting a shopping cart used to bring items to a checkout location, and inspecting its bottom tray for the presence of items, the apparatus comprising:
a. a cart-detector to detect the presence of the cart at a pre-determined location; and
b. an item-detector to detect the presence of items placed on the lower tray of the cart.
The cart detector is comprised of:
a. an arrangement of three optical sensors and corresponding retro-reflective targets; and
b. finite state-machine processing logic to discriminate a specific sequence of sensor responses.
The item detector is comprised of:
a. an optical line generator to project a structured illumination pattern on any items placed on the lower tray of the shopping cart as well as on the opposite wall of the checkout lane;
b. an area-imaging sensor and associated optics and digitizing means to capture the reflected pattern in digital form;
c. a pattern-analysis means to analyze the captured pattern; and
d. a means for transmitting the result of the pattern analysis, for example, to a system that can alert the attendant and/or the customer, or to a means that prevents further progress of the car through the checkout lane.
Therefore, in accordance with one aspect of the present invention, there is provided an apparatus for detecting a cart used to bring items to a checkout location, and for detecting an item on a bottom tray of the cart, the apparatus comprising:
a. a cart detector for detecting the cart and discriminating between the cart and other objects, the cart detector comprising:
i. a plurality of optical sensors operatively arranged within the checkout location to detect predetermined parts of the art and to produce output signals dependent on the detected predetermined parts, and
ii. electronic logic for decoding the output signals and generating an activator signal;
b. an item detector, which when activated by the activator signals, detects the item on the bottom tray and transmits an alarm signal, the item detector comprising:
i. an optical line generator and an imager that generate a digital image corresponding to the item detected; and
ii. a pattern-recognition means to process and analyze the digital image and generate the alarm signal when the item is detected.
In accordance with a second aspect of the present invention, there is provided a method of detecting a cart used to bring items to a checkout location, and an item on a bottom tray of the cart, the method comprising the steps of:
a. operatively arranging a plurality of optical sensors within the checkout location;
b. detecting predetermined parts of the cart with the optical sensors and producing output signals dependent on the detected predetermined parts;
c. decoding the output signals using electronic logic and generating an activator signal to activate an item detector that comprises an optical line generator, an imager and a pattern-recognition means;
d. generating a digital image corresponding to the item detected on the bottom tray; and
e. processing and analyzing the digital image and generating the alarm signal when the item is detected.